An oxide semiconductor film composed of a metal composite oxide has a high degree of mobility and excellent visible light transmission, and is used as a switching element, a driving circuit element or the like for a liquid crystal display, a thin film electroluminescence display, an electrophoresis display, a moving powder display, etc.
As the oxide semiconductor film composed of such a metal composite oxide, a zinc oxide-based crystalline thin film (Patent Document 1) has attracted attention. However, zinc oxide has a defect that it lacks stability, and hence, a very few of them were put into practical use.
Non-patent Document 1 discloses a thin film transistor using an amorphous transparent semiconductor film composed of indium oxide, gallium oxide and zinc oxide. However, since this transparent semiconductor film is amorphous, it has poor stability.
In addition, the above-mentioned indium oxide contains a lot of oxygen deficiencies in its crystal, and therefore, is used as a transparent material improved in conductivity. However, when indium oxide is used as a material of an oxide semiconductor film, the amount of oxygen deficiency cannot be controlled. As a result, indium oxide becomes a conductive material, making the use thereof as a semiconductor of an oxide semiconductor film difficult.
The above-mentioned oxide semiconductor film composed of a metal composite oxide has further disadvantages that the field effect mobility is as low as 8 cm2/V·sec or less and the on-off ratio of the current is small. In addition, a sputtering target used in producing the above-mentioned oxide semiconductor film has poor conductivity, and hence, a DC sputtering method, which is industrially advantageous, cannot be used, and hence, only a RF sputtering method can be used in producing the above-mentioned oxide semiconductor film. For these reasons, these oxide semiconductor films are not suited to practical use.
Furthermore, an active matrix-type image display apparatus such as a LCD (liquid crystal display) and an organic EL (Electro Luminescence) display has been widely used in view of display performance, energy saving or the like. In particular, it has come to almost constitute the mainstream as displays of cellular phones, PDAs (Personal Digital Assistant) and personal computers, laptop computers and TVs. Generally, a TFT (field-effect type thin film transistor) substrate is used in these displays.
For example, a liquid crystal display has a configuration in which a display material such as liquid crystal is filled between a TFT substrate and an opposing substrate, and a voltage is selectively applied to this display material for each pixel. Here, a TFT substrate means a substrate on which a TFT using a semiconductor thin film (also referred to as a semiconductor film) such as an amorphous silicon thin film or a polycrystalline silicon thin film is arranged as an active layer. The above-mentioned image display apparatus is driven by the active matrix circuit of a TFT. Since a TFT is arranged in the shape of an array, a TFT substrate is also referred to as a “TFT array substrate”.
Meanwhile, in a TFT substrate used for a liquid crystal display or the like, a set of a TFT and one pixel of the screen of a liquid crystal display (this set is referred to as one unit) are arranged longitudinally and laterally on a glass substrate. In a TFT substrate, for example, a gate wire is arranged longitudinally at equal intervals on a glass substrate and a source wire or a drain wire are arranged laterally at equal intervals. In addition, a gate electrode, a source electrode, and a drain electrode are respectively formed in the above-mentioned unit which constitutes each pixel.
A transistor using the above-mentioned silicon thin film is produced by using a silane-based gas, and hence, it is disadvantageous in respect of safety or equipment cost. In addition, an amorphous silicon thin film has an electron mobility which is as low as about 0.5 cm2/V·s when used in a TFT. In addition, since an amorphous silicon thin film has a small band gap, it may absorb visible rays to cause malfunction. Moreover, a polycrystalline silicon thin film requires a heating process which is conducted at relatively high temperatures, needs a large amount of energy cost, and it is hard to be formed directly on a large-sized glass substrate.
Under such circumstances, a TFT using an oxide semiconductor thin film in which a film can be formed at low temperatures has been actively developed. With the development of the above-mentioned TFT, a semiconductor device or the like using an oxide semiconductor thin film has also been developed. Further, in order to obtain a flat panel display (FPD) which is thinner, lighter in weight and higher in resistance to breakage, an attempt has been made to use a resin substrate or the like which is light in weight and flexible instead of a glass substrate.
For example, Patent Document 2 discloses a technology of a thin film transistor using an oxide semiconductor film composed mainly of zinc oxide which can be formed at low temperatures.
In addition, Patent Document 3 discloses a technology of a field effect transistor using as an active layer (channel layer) an amorphous oxide containing at least one of In, Zn and Sn.
However, the thin film transistor in Patent Document 2 is required to be improved in transparency or electric properties as a transistor or the like.
In addition, since the transparent semiconductor film used in the channel layer is amorphous, the field effect transistor of Patent Document 3 has problems that the characteristics thereof may change largely with time or by heat, or the threshold voltage may change greatly when used for a long period of time. In particular, in the production process, if a heat of 300° C. or higher is applied, for example, thermal changes in the characteristics thereof has become a great obstacle for industrialization. One of the reasons is that the number of careers is too large or that the film is amorphous. Another reason is that the move of oxygen tends to occur easily and the carrier concentration tends to change easily since oxygen is forced to be included in order to increase the partial oxygen pressure at the time of film forming.
In addition, it is difficult to control an amorphous transparent semiconductor thin film, since a large amount of oxygen tends to be introduced at the time of film formation. As a result, the carrier concentration tends to change with the passage of time or by environmental temperatures. For this reason, it is necessary to control accurately the oxygen partial pressure at the time of film formation. Therefore, an amorphous transparent semiconductor film is defective in reproducibility, stability or the like for industrialization.
Furthermore, since this transparent semiconductor membrane is amorphous, its resistance to chemicals such as an etching solution, the representative example of which is PAN, is low, a metal wiring on the semiconductor film cannot be subjected to wet etching. Moreover, it has a defect that the refractive index is large and light transmittance of a multilayer film is lowered easily. In addition, since this transparent semiconductor thin film is amorphous, it may adsorb oxygen, water, or the like in an atmospheric gas, whereby electrical properties of the film may be changed and thus the yield may be lowered.
That is, an amorphous oxide has a problem that control of electron career density is difficult, and hence it is poor in stability, uniformity, reproducibility, heat resistance and durability.
Patent Document 1: JP-A-2004-273614
Patent Document 2: JP-A-2003-298062
Patent Document 3: JP-A-2006-173580
Non-Patent Document 1: NATURE, vol. 432, p488-492, (2004)
An object of the invention is to provide a sputtering target which can be crystallized at relatively low temperatures and is capable of producing an oxide semiconductor film having stable semiconductor properties.
Another object of the invention is to provide a sputtering target which is improved in conductivity and can form a film by using a DC sputtering method.
The invention has been made in view of the above-mentioned problems, and is aimed at providing a semiconductor device improved in transparency, electric properties, stability, uniformity, reproducibility, heat resistance, durability or the like.